The present invention is directed generally to an agitator mill used to grind a product with a grinding media and, in particular, to an agitator mill having a freely rotatable separator screen which is caused to rotate through the fluid coupling between the screen and the rotating agitator shaft by the slurry.
Agitator mills are used generally to disperse solids such as pigments in a liquid. The grinding and mixing is performed in the chamber of the agitator mill which includes an agitator shaft with discs, pegs or the like. The shaft is driven by a mechanical device such as a motor. A grinding media, such as sand or the like, in the agitator milling chamber is used to disperse the solid material in the liquid. After the grinding and mixing of the solids and liquid is complete, it is necessary to separate the mixture from the grinding media and then to discharge the mixture from the milling chamber.
Over the years, various types of separator screen devices have been proposed for separating the ground mixture from the grinding media before discharge of the mixture. In the prior art, there are two types of separator screen constructions. In a first version, the separator is driven either by direct coupling to the agitator shaft or separately driven by a separate motor or other such device. In a second version, such as that disclosed in U.S. Pat. No. 4,620,673 to Canepa, et al., the separator screen is mounted to be non-rotatable.
The Canepa, et al. construction provides an agitator shaft with an end having a cavity therein. The separating device extends fully into the cavity such that the agitator shaft rotates therearound while the separating device remains stationary. The static design of the separating device in Canepa, et al. cannot yield to the fluid force or act to pump the material. The screen is subject to hydraulic packing as the ground product and grinding media are forced thereagainst. This may lead to both clogging of the screen and premature wear thereof.
Agitator mills which include separator screens which are driven by the agitator shaft tend not to be as effective in the milling and separating function. Agitator mills which include rotating separator screens which are driven by a separate motor suffer the problems inherent when additional mechanical devices are utilized.
Accordingly, it is desired to provide an agitator mill which overcomes the problems inherent in the prior art by providing a construction with a freely rotatable separator screen which is driven by the fluid coupling with the agitator shaft through the slurry.